Thanks for contacting us, that sounds like an interesting project.
RailClone uses discrete units of geometry, created using standard modelling tools, and combines them into complex one and two dimensional arrays driven by splines. These can then be combined, sequenced and selected using conditional rules to create a finished style. You can read a little more about RailClone's approach to modelling here
Because RailClone combines and deforms existing objects rather than generating geometry from scratch, it isn't currently able to transition between different cross sections automatically like the Loft or Sweep objects.
However, depending on the project, a workaround might be to model the bridge straight and in small chunks and then use RailClone to deform it along a spline. In this example I've broken up the bridge into straight sections and transitions. The red segments are the transitions, which can be places at regular intervals or by targeting vertices on the spline. The blue sections will automatically scale between the transition pieces.
This may not suit your particular scenario and it does require a little modelling upfront, but once built making changes should be simple. A setup like this would probably require RailClone Pro, as the lite version is limited to 3 segments.
I hope that helps,
if you have further questions, please let me know.